002_pg_upgrade.pl: Move pg_dump test code for better stability

002_pg_upgrade.pl: Move pg_dump test code for better stability

The alleged "statistics pg_dump bug" that prevented us from enabling
stats dumping in commit 172259afb563 wasn't a pg_dump bug after all: it
was just a side effect of not running pg_dump at the right time (namely,
before giving autovacuum some time to do its thing and then disabling it
to stabilize things).  Move the code around to fix this problem and
enable statistics dumping.

Author: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Diagnosed-by: Jeff Davis <pgsql@j-davis.com>
Discussion: https://postgr.es/m/5f3703fd7f27da62a8f3615218f937507f522347.camel@j-davis.com
Discussion: https://postgr.es/m/CAExHW5sDm+aGb7A4EXK=X9rkrmSPDgc03EdADt=wWkdMO=XPSA@mail.gmail.com